Phonon-assisted normal incidence intersubband absorption in semiconductor quantum wells

In the presence of a normally incident mid-IR pulsed laser field, phonon-assisted photon absorption by both intrasubband and intersubband phonon scattering of conduction electrons in GaAs/AlGaAs quantum wells are predicted. The novel non-resonant and non-linear intersubband absorption is found by including the photon-induced phonon scattering process in a Boltzmann equation for phonon energies smaller than the energy separation between two electron subbands in the quantum well. The predicted phonon-assisted photon absorption by intersubband transitions of electrons from the first to the second subband is a unique feature in quantum-well systems and is expected to have a significant effect on the electron populations in both subbands.     

Photoinduced infrared spectroscopy of bound-to-bound and bound-to-continuum transitions in SiGe/Si quantum wells

Infrared spectroscopy of intersubband transitions in the valence band of undoped SiGe/Si quantum wells is presented. Optical pumping of interband transitions is used to generate carriers in the wells. The spectral features of bound-to-bound and bound-to-continuum transitions are analyzed and compared to those of GaAs quantum wells. In samples with only one heavy hole bound level, a ratio of 20:1 is observed between intersubband and free carrier absorption. Room temperature photo-induced absorption is only observed in samples with high germanium content (≈50%). The feasibility of normal-incidence infrared modulators based on s-polarized intersubband absorption is also demonstrated. Resonant dispersion associated with intersubband transitions is evidenced.     

Cylindrical GaAs quantum wires incorporated within chrysotile asbestos nanotubes: fabrication and polarized optical absorption spectra

GaAs has been injected into chrysotile asbestos channels. GaAs quantum wires (nanocylinders) with diameters ∼ 6 nm have been observed in the channels by means of transmittance electron microscopy. Polarized optical absorption spectra of asb-GaAs (chrysotile asbestos containing GaAs wires) have been studied. A high anisotropy of the absorption has been observed, and intersubband transitions in the visible light region due to a strong quantum size effect have been found.     

The electric field effects on intersubband optical absorption of Si δ-doped GaAs layer

The intersubband transitions in Si δ-doped GaAs structures is theoretically investigated for different applied electric fields. For an uniform distribution the electronic structure has been calculated by solving the Schrödinger and Poisson equations self-consistently. From our calculations, it is found that the subband energies and intersubband optical absorption is quite sensitive to the applied electric field. This gives a new degree of freedom in various device applications based on the intersubband transitions of electrons.     

Non-linear optical processes involving excited subbands in laser-dressed quantum wires with triangular cross-section

The conduction subband structure of a triangular cross-section GaAs/AlGaAs quantum well wire under intense laser field is theoretically investigated by taking into account a finite confining potential. The calculation of the subband energy levels is based on a two-dimensional finite element method within the effective mass approximation. It is shown that a transversally polarized laser field non-uniformly shifts the subband energy levels and could be used for tuning the intersubband transitions and altering the related optical susceptibilities. We found that the non-resonant laser field allows the magnification and the red- or blueshift of the third-order non-linear susceptibility peaks for particular polarizations of the pump light and proper laser parameter values. The effects of the laser dressing field on the intersubband third harmonic generation and quadratic electro-optical process are discussed.     

The effect of hydrostatic pressure on subband structure and optical transitions in modulation-doped quantum well

Within the framework of effective-mass approximation, we have calculated theoretically the effects of hydrostatic pressure and doping concentration on subband structure and optical transitions in modulation-doped GaAs/Al_xGa_1−xAs quantum well for different well widths. The electronic structure of modulation-doped quantum well under the hydrostatic pressure is determined by solving the Schrödinger and Poisson equations self-consistently. The results obtained show that intersubband transitions and the subband energy levels in the modulation-doped quantum well can be significantly modified and controlled by the well width, donor concentration and hydrostatic pressure.     

Dispersive Properties of tunnelling—induced transparency in an asymmetric double quantum well

We have investigated the dispersive properties of tunnelling-induced transparency in asymmetric double quantum well structures where two excited states are coupled by resonant tunnelling through a thin barrier in a three-level system of electronic subbands. The intersubband transitions exhibit high dispersion at zero absorption, which leads to the slow light velocity in this medium as compared with that in vacuum (c=3×10⁸). The group velocity in a specific GaAs/AlGaAs sample is calculated to be v_g=c/4.30. This structure can be used to compensate for the dispersion and energy loss in fibre optical communications.     

Optical absorption of a quantum well with an adjustable asymmetry

The effects of asymmetry and the electric field on the electronic subbands and the nonlinear intersubband optical absorption of GaAs quantum wells represented by a P?schl-Teller confining potential are studied. The potential itself can be made asymmetric through a correct choice of its parameter set and this adjustable asymmetry is important for optimizing the absorption. In that way optimal cases can be created. We calculate the modified wave functions and electronic subbands variationally. The linear and the nonlinear optical intersubband absorption coefficients are calculated. Numerical results for a typical GaAs quantum well are presented. The nonlinear part of the absorption coefficient is strongly modified by the asymmetry parameters while the electric field affects it at smaller values of the parameters. - 78.67.De Quantum wells     

Phase-resolved nonlinear response of a two-dimensional electron gas under femtosecond intersubband excitation

Strong electric-field transients resonant to intersubband transitions in n-type modulation-doped GaAs/AlGaAs quantum wells induce coherent Rabi oscillations, which are demonstrated by a phase-resolved measurement of the light emitted by the sample. The time evolution of the intersubband polarization is influenced by Coulomb-mediated many-body effects. The subpicosecond period and the phase of the Rabi oscillations are controlled by the properties of the midinfrared driving pulse.     

Effects of an intense, high-frequency laser field on the intersubband transitions and impurity binding energy in semiconductor quantum wells

Within the framework of effective-mass approximation, using a variational method, the effect of high-frequency laser field on intersubband transitions and the binding energy of shallow-donor impurities in a semiconductor quantum well are investigated. We have found that the increase of the laser-dressing parameter leads to important effects on the electronic and optical properties of a quantum well. This gives a new degree of freedom in various device applications based on the intersubband transition of electrons.     

Photoionization cross section of a donor impurity in a two-dimensional anisotropic quantum dot

A system of an electron with a hydrogenic impurity confined in a two-dimensional anisotropic quantum dot has been investigated. We report a calculation for the binding energy of a donor impurity. The important feature of a donor impurity in a two-dimensional anisotropic quantum dot is obtained via an analysis of the binding energy. The photoionization cross section associated with intersubband transitions has been calculated. The results are presented as a function of the incident photon energy. The results show that the photoionization cross section of a donor impurity in a two-dimensional anisotropic quantum dot is strongly affected by the degree of anisotropy and the size of the quantum dot.     

Intersubband optical absorption coefficient changes and refractive index changes in a two-dimensional quantum pseudodot system

The optical absorption coefficient changes and refractive index changes associated with intersubband transitions in a two-dimensional quantum pseudodot system under the influence of a uniform magnetic field are theoretically investigated. In this regard, the electronic structure of the pseudodot system is studied using the one-band effective mass theory, and by means of the compact density matrix approach linear and nonlinear optical absorption coefficient and refractive index changes are calculated. The effects of an external magnetic field and the geometrical size of the pseudodot system on the optical absorption coefficient and refractive index changes are investigated. It is found that the absorption coefficient and refractive index changes are strongly affected not only by an external magnetic field but also by the geometrical size of the pseudodot system.     

Optical intersubband transitions in double Si -doped GaAs under an applied magnetic field

For different applied magnetic fields, the intersubband transitions of double Si δ-doped GaAs structures is theoretically investigated for a uniform donor distribution. The electronic structure has been calculated by solving the Schrödinger and Poisson equations self-consistently. It is found that the intersubband optical absorption and mobility are sensitive to the applied magnetic field: for all allowed intersubband transitions the intersubband absorption spectra show blueshifts. The results open the possibility to design devices for use as optical filters controlled by an applied magnetic field, depending on the δ-doped structure. It is hoped that these results will provide important improvement in device applications, for a suitable choice of magnetic field.     

Modulation spectroscopy study of an InGaAs/GaAs-strained asymmetric triangular quantum well heterostructure

Il Nuovo Cimento D - We have studied modulation spectra related to the intersubband transitions at 300 K and 20 K from an In GaAs/GaAs-strained asymmetric triangular quantum well (ATQW)...     

THz-field-induced electronic transmission step-structure for a quantum wire

We study theoretically the low-temperature electronic transport property of a straight quantum wire under the irradiation of a finite-range transversely polarized external terahertz (THz) electromagnetic (EM) field. Using the freeelectron model and the scattering matrix approach, we show an unusual behaviour of the electronic transmission of this system. A sharp step-structure appears in the electronic transmission probability as the EM field strength increases to a threshold value when a coherent EM field is applied. We demonstrate that this effect physically comes from the inelastic scattering of electrons with lateral photons through intersubband transitions.     

Hole intersubband transitions in GaAs/AlGaAs multiple quantum wells… A new method for determining the Luttinger parameters

Resonant electronic Raman scattering from photoexcited holes has been observed for multiple quantum wells (MQW's) grown in the [111]b and [100] directions. The measurements indicate that the heavy hole mass in the [111] direction is 0.75mo. This value is 2.2 times larger than the value characteristics of the [100] direction. The measurements also quantify the degree of anisotropy for the light holes. We propose a new set of Luttinger parameters that describe the anisotropy of the valence band in GaAs and are consistent with the interband and intersubband transitions observed in [100] and [111]b MQW's.     

Strong correlation of electronic and lattice excitations in GaAs/AlGaAs semiconductor quantum wells revealed by two-dimensional terahertz spectroscopy

Coulomb-mediated interactions between intersubband excitations of electrons in GaAs/AlGaAs double quantum wells and longitudinal optical phonons are studied by two-dimensional spectroscopy in the terahertz frequency range. The multitude of diagonal and off-diagonal peaks in the 2D spectrum gives evidence of strong polaronic signatures in the nonlinear response. A quantitative theoretical analysis reveals a dipole coupling of electrons to the polar lattice that is much stronger than in bulk GaAs, due to a dynamic localization of the electron wave function by scattering processes.     

Electron Raman scattering of a two-dimensional pseudodot system

We have carried out the theoretical investigation of the differential cross section for the electron Raman scattering process, which is associated with intersubband transitions in a two-dimensional quantum pseudodot system. The great advantage of our methodology is that it enables confinement regimes by varying two parameters in the model potential. It is found that the differential cross section is affected by the external magnetic field, the geometrical size and the chemical potential of the pseudodot system.     

Coupled electron-phonon modes in optically pumped resonant intersubband lasers

Intersubband lasing at 12-16 microm based on a CO2 laser pumped stimulated resonant Raman process in GaAs/AlGaAs three-level double-quantum-well structures is reported. The presence, or lack of, lasing action provides evidence for resonantly coupled modes of collective electronic intersubband transitions and longitudinal optical phonons. An anticrossing behavior of these modes is clearly seen when the difference between the pump and lasing energies (i.e., Stokes Raman shift) is compared with the subband separation. This work reveals the significance of the strong coupling between intersubband transitions and phonons and raises a new possibility of realizing a phonon "laser."     

Birefringence and absorption of infrared radiation in tunnel-coupled GaAs/AlGaAs quantum wells in a longitudinal electric field

Birefringence and absorption modulation under electron heating in a longitudinal electric field in the tunnel-coupled GaAs/AlGaAs quantum wells have been found and investigated in the spectral region corresponding to intersubband electron transitions. The observed phenomena are explained by electron heating in electric field and electron transfer in real space. The equilibrium absorption spectra at different lattice temperatures are analyzed.